**1. Introduction**

Surface solutions using PVD (Physical Vapor Deposition) coatings for the improvement of the wear resistance and tribological properties of tools and components started on an industrial scale around the middle of the 1970s in the Soviet Union. The first component coating was MoN [1]. The first successful industrial coating for tooling applications in Western Europe was the "golden" TiN coating applied by low-voltage arc evaporation developed at Oerlikon Balzers [2]. A modification of this coating with carbon followed, and the TiNC coating types were implemented in mass production at the end of the 1980s. In parallel, Cr-based coatings were also introduced, including modifications using oxygen and carbon [3]. Besides nitride, carbonitride and oxynitride coatings, hard amorphous carbon coatings, the DLC (Diamond Like Carbon) coatings, were developed and applied as well [4,5]. These coatings were deposited using several coating architectures [6]. Many coating developments focused on alloying TiN using different additional elements [5].

The most important industrial development after TiN, CrN and DLC was achieved in the Ti-Al-N system [7]. Around the year 2000, this coating family was the dominant coating, especially for cutting tools [8,9]. However, in the Ti-Al-N system, only minor improvements of coating properties and the performance of cutting tool applications were possible, e.g., by alloying with Si, Y, Cr, C, O [10–13]. A big step forward was achieved through the investigation of totally new coating compositions. This led to the development of a new coating generation based on Al-Cr-N. Coatings in the AlCrN system span the compositional range from Al-rich to Cr-rich. In this paper, the terminology CrAl-based is used for coatings with a Cr content that is higher than that of Al (Cr/Al > 1 in at.%), and conversely, AlCr-based is used for coatings in which Al/Cr > 1.


**Citation:** Vetter, J.; Eriksson, A.O.; Reiter, A.; Derflinger, V.; Kalss, W. Quo Vadis: AlCr-Based Coatings in Industrial Applications. *Coatings* **2021**, *11*, 344. https://doi.org/ 10.3390/coatings11030344

Academic Editor: Armando Yáñez-Casal

Received: 12 February 2021 Accepted: 15 March 2021 Published: 18 March 2021

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Results dealing with coatings with a low Al content (Al/(Al + Cr) ratio < 0.5) and (CrAl)N coatings, deposited by sputtering, were published for the first time in 1990 [14,15]. In the same year, coatings with an Al/(Al + Cr) composition of about 0.5 deposited using a modified hollow cathode discharge process were also presented [16]. Arc evaporation was used to deposit (AlCr)N coatings with a high Al content with an (Al/(Al + Cr) ratio of >0.5 [17,18]. The first multilayer coating with an Al content greater than the Cr content was a multilayer architecture with CrN [19]. Another big step was the development of (TiSi)N coatings [20]. Coating solutions were also developed which combined the two coating types in multilayer structures [21].

A challenge for turning tools was, and still is in part, the development of alphaalumina using PVD. A PVD solution to add metallic doping elements to aluminium was patented in 1992 [22]. Processes to deposit oxides based on arc evaporation were successfully developed on the basis of Al-Cr cathodes [23].

This paper will review the main industrial-oriented coating developments using CrAl- and AlCr-based coatings, covering a wide variety of chemical and structural fields as well as architectures. The main deposition methods used will be described briefly. The review will concentrate on arc deposition processes, although sputtering techniques, including high-power impulse magnetron sputtering (HiPIMS), will be discussed as well. The main focus is on arc deposition processes and nitride coatings with an Al content greater than the Cr content and on (AlCr)ON coatings. Coatings with a cubic structure and various multilayer coatings are used most commonly in industrial applications. Selected application examples will be presented in the last section.
